Continuously-variable-ratio transmission of the toroidal-race-rolling traction type

ABSTRACT

A roller control system for a continuously-variable-ratio transmission (CVT) of the toroidal-race rolling-traction type. The system includes an operating mechanism having a first part (5, 6) operable to control the position of the roller center along the torus center circle but incapable of defining the tilt angle (36) adopted by the roller (7), and a second part comprising a mechanical link (13) connected to the roller bearings and operable to control the tilt angle (36). The link (13) lies substantially parallel to the plane of the roller (7) and is constrained to pass through a predetermined single point (14).

This application is a continuation of International Application No.PCT/GB96/01465 filed on Jun. 19, 1996.

BACKGROUND OF THE INVENTION

This invention relates to continuously-variable ratio transmissions("CVT's") of the toroidal-race rolling-traction type and especially tomeans for controlling the orientation of the rollers in the variators,that is to say the ratio-varying components, of such CVT's.

FIG. 1 is a simplified representation of part of one known variator,viewed in a direction perpendicular to the common axis N of the twodiscs G and J. A single roller A, which will in practice be one of a setof three disposed at equal angular intervals around the axis N,transmits traction between part-toroidal races F and H in discs G and Jrespectively, and is mounted within a carriage C to rotate about acenter and axis (B) both of which are defined by and fixed relative tothe carriage. A rod P connects the carriage C to a piston D which hasfreedom to move axially within a fixed cylinder E, and also to tiltslightly as it does so without losing seal.

Such a variator has been found to work efficiently in a so-called"torque-controlled" CVT where a pressure generated hydraulically (bymeans not shown) in cylinder E exerts a force on piston D, which forequilibrium must balance the reaction force resulting from the resultanttorque at the contact between the roller A and the races F and H. RollerA changes its angle of orientation (or "tilt angle"), and thus the ratioit transmits between discs G and J, by tilting about the axis of rod P,and it has been found that each position of the center of piston D,within its range of axial movement within cylinder E, correlates with aunique tilt angle of the roller A. In other words, each equilibrium tiltangle of the roller is uniquely defined by just three points, namely thelocations of contact of the roller A with races F and H and the locationof the centre of the piston D. Such a variator, and the CVT of which itis part, is described and shown in more detail in Patent EP-B-0444086.

As is well known in this art, the center of the roller is at all timesconstrained to follow the center circle of the torus to which races Fand H conform. That center circle must lie in the mid-plane M of thetorus. Rod P, which as already noted defines the tilt axis of theroller, is inclined to that plane at an angle L, known as the castorangle. The advantages of operating a toroidal-race variator with asubstantial degree of castor angle, say of the order of 15°, are wellknown in the art.

It will be seen that in the apparatus of FIG. 1, as is typical in theprior art, the axis of movement of the component which applies thereaction force (the piston D) and the axis of tilt (the rod P) coincideat the third of the three points by which each angular setting of theroller is uniquely defined. This coincidence imposes constraints uponthe location and orientation of certain components, particularly thecylinder E, and thus on the overall dimensions of the variator. Forexample, because the axis of cylinder E is inclined to the transversemid-plane M by the castor angle L, the radius at which the cylinder islocated relative to the disc axis N exceeds the radius of the discsthemselves. If it did not, a corner of the cylinder would be at risk offouling the disc J.

SUMMARY OF THE INVENTION

According to the present invention such constraints are diminished, thusproviding greater freedom to locate the equivalent of cylinder E so asto minimise the overall dimensions of the variator, by separating thefunctions of reaction force generation and tilt control so as to avoidthe two axes coinciding at the third point, in the manner justdescribed. Such separation occurs also in the unusual CVT described inU.S. Pat. No. 3,933,054, but there the tilt control of each roller isachieved by a shaped slot, in which runs a pin fixed to the rollerbearings and projecting in a direction coaxial with the roller itself.Such a construction has two particular disadvantages. Firstly that anypin-and-slot engagement is inherently liable to wear and friction.Secondly that the orientation and minimum length of the pin, relative tothe roller, is such that whenever the two discs tend to move slightly inunison in either direction along their common axis, as they tend to docontinually in use under varying loads, the consequent effect of thepin-and-slot engagement will be to cause the roller to tend to "steer"to a new tilt angle, thus changing the transmitted ratio although nosuch change has been demanded. The present invention relies on adifferent tilt control mechanism, which requires no pin-and-slotconnection and which is inherently less prone to cause any ratio changein response to axial movements of the discs.

BRIEF DESCRIPTION OF THE DRAWING(S)

The present invention is defined by the claims, the contents of whichare to be read as included within the description of this specification,and the invention will now be described by way of example with referenceto the following further figures of diagrammatic drawings in which:

FIG. 1 is a prior art viewed in a direction perpendicular to the commonaxis of discs;

FIG. 2 shows one roller control mechanism. viewed along the axis of theroller;

FIG. 3 is a partial view in the direction of the arrow III in FIG. 2;

FIG. 4 is another partial view, in the direction of the arrow IV in FIG.2; and

FIG. 5 shows part of another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 2 shows a source 1 of hydraulic power connected, by way of conduits3, 4 and of a central programmed control system 2 of a kind appropriateto torque control, to opposite ends of a cylinder 5 containing a piston6. The roller 7 is rotatably mounted within a carriage 8, which isattached to the piston 6 by way of a rod 9. Items 7, 8, 9 and 6correspond with items A, C, P and D in FIG. 1 to the extent that thehydraulic forces generated within cylinder 5 are controlled so as tobalance the torque reaction forces generated at the contacts between theroller 7 and discs 10 and 11, so bringing the CVT into equilibrium withthe roller at the tilt angle appropriate to that torque reaction. InFIG. 1, however, both the center of roller A and its axis of rotationwere fixed relative to carriage C. In FIG. 2, as will be furtherexplained, the carriage 8 determines only the center of rotation of theroller. Its axis of rotation, and thus also the perpendicular tilt axisabout which it tilts to change ratio, is determined by a rod 13 whichcan slide through, but is radially constrained by, an aperture 14 formedin a member 15 which is either fixed to or part of the fixed structureof the CVT. In FIGS. 2 and 3 it is shown fixed to cylinder 5.

FIG. 3 shows that piston 6 comprises a two-part cylindrical sleeve 17,the ends 18, 19 of which pass through sealed apertures in the oppositeend walls of cylinder 5. The sleeve 17 also has a central part 20 ofenlarged diameter, which carries a seal 21 and thus moves as a pistonwithin the central chamber 22 of cylinder 5. Within the central part 20of the sleeve 17 fits a ring 25, in the middle of which a member 26 isfree to rotate in the manner of a spherical bearing. Rod 9 is fixed tomember 26. This form of piston is thus essentially of a kind shown inmore detail in patent publication WO 92/11475 and has the advantage thatthe rod 9 is free to rotate through a conical angle but is out ofcontact with the hydraulic fluid in cylinder 5, so avoiding the needthere would be for flexible seals around the rod if it were attached toa conventional piston, and was therefore exposed to the cylinder fluid.

As FIG. 3 also shows, carriage 8 supports roller 7 by way of a rod 28 towhich is fixed the inner half 29 of a spherical bearing. Thus, as hasalready been said, carriage 8 fixes only the centre and not also theaxis of rotation of roller 7. As FIGS. 3 and 4 together show, the outerhalf 30 of the spherical bearing is held by the inner wall of a sleeve31 which is attached (by way of a second carriage 16 which fits withgood clearance within the jaws of carriage 8) both to the rod 13 and toa ball race 32 about which the roller 7 spins.

Because of the two spherical joints at 25/26 and 29/30 thepiston/cylinder combination 5/6, which can be regarded as a first partof the roller operating mechanism, can exert thrust on the roller 7 tobalance its reaction forces against the discs 10 and 11, but cannotdefine the ratio angle adopted by the roller. In contrast the rod 13 andassociated parts, together constituting a second part of the operatingmechanism, can exert no such thrust, but define both the axis aboutwhich the roller tilts to change ratio and the angle (the "castor angle"36) which that axis makes with the torus mid-plane 35.

In terms of the explanation of the operation of a torque-controlled CVTgiven in the third paragraph of this specification, in FIG. 2 eachequilibrium tilt angle of the roller 7 now correlates with a uniquetriangle of which the fixed aperture 14 is always the vertex, but thelocations of the two roller/disc contacts and the distances between eachof those contacts and the vertex are unique. It should also be notedthat the axes of rod 13 and roller 7 are coplanar and intersect at theroller center, so reducing to a minimum any steer effect that the rodwill impose on the roller in response to axial movements of the discs10, 11 under load.

In FIG. 2 the axis of cylinder 5 is shown aligned with the torusmid-plane 35: alternatively, it could for instance be offset from thatplane but parallel to it. Because the cylinder axis is no longer alignedwith the roller tilt axis (as it is, for instance, in the detailedembodiments of EP-B-0444086) the structure of cylinder 5 can now belocated with more freedom, and notably at a substantially smaller radiusrelative to the common axis of the discs, and the relative simplicity ofthe rod 13 and the other parts that define the tilt axis enable them tobe located at a smaller radius still. Possibly, as indicated in FIG. 2,the entire structure of the cylinder 5, the rod 13 and the member 15 maybe accommodated within the imaginary cylinder of which the two discs 10and 11 constitute the end walls.

In the alternative construction shown in outline in FIG. 5 the aperture14, instead of being formed in a member 15 fixed to the structure of theCVT as a whole, is now formed in a flange 40 formed at the forward tipof end 18 of the sleeve 17. The aperture 14 therefore moves with thepiston 6, and each equilibrium tilt angle of the roller 7 (not shown inFIG. 5) correlates with a trio of unique locations, namely the tworoller/disc contacts and the instantaneous location of the aperture 14,which of course now reflects the position within its stroke of thepiston 6. With this embodiment, it is necessary to ensure that theaperture 14 follows a predetermined path as the piston 6 moves back andforth. This would not be assured if the piston 6 were of conventional,circular outline and were free to rotate, as well as move axially,relative to the cylinder 5. Such rotation can be prevented, and thepredetermined movement of aperture 14 therefore assured, by a guide pin41 projecting axially forward from the structure of the cylinder 5 andpassing through a second aperture 42 in the flange 40. Alternativelyrotation could be prevented by making the end 18 of sleeve 17 ofnon-circular outline where it passes through the front wall of thecylinder 5.

We claim:
 1. A roller control system, for a continuously-variable-ratiotransmission, comprising a pair of spaced apart discs defining a torusmid-plane therebetween; the roller control system further including aroller being located between the pair of spaced apart discs, the rollerhaving a tilt angle, a roller rotational plane, roller bearings tofacilitate rotation of the roller, and the roller bearing being locatedat a roller center; and an operating mechanism for controlling aposition of the center of the roller along the torus mid-plane and forcontrolling the tilt angle of the roller;wherein said operatingmechanism has: a first mechanical link operable to control said positionof the roller center along said torus mid-plane but incapable ofcontrolling said tilt angle of the roller; and a second mechanical linkconnected to said roller bearings and operable to control the tilt angleof the roller, and the second mechanical link lies substantiallyparallel to the plane of the roller and is constrained to pass through apredetermined single point fixed relative to a structure of thetransmission.
 2. The roller control system according to claim 1, whereinthe first mechanical link comprises a piston movable along an axisparallel to the torus mid-plane.
 3. The roller control system accordingto claim 2, wherein the piston has a piston axis and said piston axislies in the torus mid-plane.
 4. The roller control system according toclaim 2, wherein the piston includes a piston axis and the piston isprevented from rotating about the piston axis.
 5. The roller controlsystem according to claim 1, wherein the second mechanical link issubstantially coplanar with the roller.
 6. The roller control systemaccording to claim 1, wherein the second mechanical link of theoperating mechanisms lies at no greater radius, from the center of theroller than the first mechanical link.
 7. The roller control systemaccording to claim 1, wherein the second mechanical link of theoperating mechanism lies at no greater radius, from the center of theroller than the first mechanical link and both parts of the operatingmechanism lie wholly within the radius of the discs between which theroller is transmitting traction.